I have set of stairs that leads from the main floor to the basement. A little over half way down there is a landing (typical L-shape). I'm looking to put in an insulated subfloor in my basement, which could raise the floor height quite a bit. If I did so, I might have to change the bottom set of stairs. I was thinking of cutting new stringers, which would allow my steps to all be of uniform height, but that would make the risers on the bottom set of stairs differ from the upper set. I have 6 steps, and if I were to put in a 3 1/4" sub-floor [1"XPS + 2x4 (1.5") sleepers + 3/4" plywood/OSB], that'd change the riser height by more than 3/8". Is this a problem?
No it is separated by a landing so it doesnt matter. Just as long as the rise is between 4-7 3/4 inches.
According to the International Residential Code (IRC), the 3/8" variance only applies to a "flight of stairs" not the entire "stairway".
International Residential Code 2009
Chapter 3 - Building Planning
Section 311 Means of Egress
R3220.127.116.11 Riser height. The maximum riser height shall be 73/4 inches (196 mm). The riser shall be measured vertically between leading edges of the adjacent treads. The greatest riser height within any flight of stairs shall not exceed the smallest by more than 3/8 inch (9.5 mm).
Looking at the definition of stairway and flight, you can see that the the 3/8" variance only applies to individual flights of stairs, and not the entire stairway.
Chapter 2 - Definitions
Section 202 Definitions
Stairway. One or more flights of stairs, either exterior or interior, with the necessary landings and platforms connecting them, to form a continuous and uninterrupted passage from one level to another.
Flight. A continuous run of rectangular treads or winders or combination thereof from one landing to another.
However, you may find that changing the riser height by too much within a stairway could lead to a tripping hazard. As you travel up/down the stairs, your body gets used to the step height. If midway through the stairway the step height changes, you may become temporarily disoriented and potentially trip or misstep.
If you have a high ceiling, you could put in a 7" subfloor, and just lose the last step.